We present the results of VLT optical spectroscopy of a complete sample of 78 EROs with R - Ks \geq 5 over a field of 52 arcmin ^ { 2 } .
About 70 % of the 45 EROs with Ks \leq 19.2 have been spectroscopically identified with old passively evolving and dusty star-forming galaxies at 0.7 < z < 1.5 .
The two classes are about equally populated and for each of them we present and discuss the average spectrum .
From the old ERO average spectrum and for Z = Z _ { \odot } we derive a minimum age of \sim 3 Gyr , corresponding to a formation redshift of z _ { f } \buildrel > \over { \sim } 2.4 .
PLE models with such formation redshifts well reproduce the density of old EROs ( consistent with being passively evolving ellipticals ) , whereas the predictions of the current hierarchical merging models are lower than the observed densities by large factors ( up to an order of magnitude ) .
From the average spectrum of the star-forming EROs we estimate a substantial dust extinction with E ( B - V ) \buildrel > \over { \sim } 0.5 .
The star formation rates , corrected for the average reddening , suggest a significant contribution from EROs to the cosmic star-formation density at z \sim 1 .